The present invention is concerned with a pressure supply system especially provided for generating the auxiliary pressure of a hydraulic brake system for motor vehicles. The system comprises an accumulator, a hydraulic pump driven by electromotic force, and a pressure switch which, by way of an interconnected motor relay, at a predetermined lower pressure limit, turns on the pump motor and, at a predetermined upper pressure limit, turns the pump motor off.
It is known in the art to automatically control the pressure in the auxiliary pressure supply system of a hydraulic brake system with the aid of a pressure switch and to maintain the pressure within the operating pressure range. Such a system, for example, if formed of a hydraulic reservoir charged and recharged with the aid of a hydraulic pump actuated by electromotive force. The pressure switch monitors at least two or, when combined with a pressure warning system, three switch points. In a system intially non-pressurized, first a warning lamp will flash until the lowest pressure value is reached. Thereafter, loading of the reservoir is continued until an upper switch point or an upper pressure limit value is reached. The pump, thereupon is reswitched only when, due to unloading of the reservoir, the pressure has dropped to a lower pressure limit value which is clearly below the upper threshold value. By this "hysteresis", a frequent turn on and off of the pump is avoided. However, it has to be ensured that the pressure remains within the limits and, hence, within the operating pressure range.
In conventional pressure switches and pressure warning switches of this type, the pump driving motor is turned on and off by way of a relay. However, in practice, substantial difficulties are encountered because, during the charging and decharging operation, the pressure slowly approaches the switch point. Although efforts have been taken to attain, by mechanical means, an abrupt-change characteristic or an abrupt change-type switch-over from one switching position into another switching position, frequently, when the switch point is approached, an instable or indifferent state of equilibrium occurs. If defective conditions, such as mechanical shocks, are added, which cannot be precluded in a motor vehicle, this will result in a fluttering of the switches or in an excitation of vibrations causing highly disadvantageous effects, with the consequence of increased wear. In vehicles including electronic systems, electromagnetic interference waves are generated. All measures taken to improve the switch characterisic require substantial mechanical efforts.
It is, therefore, an object of the present invention to overcome the shortcomings involved with conventional pressure supply systems and described and to develop a pressure switch suitable for such systems which is easy to manufacture and which, with a very slow change in the auxiliary pressure, ensures that the pump motor is turned on and off in an abrupt change-type manner.